1. Field of the Invention
The present invention relates to a vector modulator in a mobile communication system.
2. Background of the Related Art
In general, a Radio Frequency (RF) system changes a frequency, or controls an amplitude and a phase of a signal. The signal denotes a quantity (for example, current, voltage, frequency, etc.) which can be specifically observed. The modulation is a form of a signal suitable for transmission to remote destination in a noiseless state. In the RF field, amplifiers (which are RF control devices), attenuators, phase shifters, and switches are used for changing an amplitude and phase of a received signal, thereby controlling the received signal. The vector modulator, which is another such RF signal control device, has an active device and a passive device combined, and is suitable for controlling the amplitude and the phase on the same time.
FIG. 1 illustrates a block diagram showing a system of a related art vector modulator. Referring to FIG. 1, a quadrature hybrid coupler 11 is provided for receiving a RF signal through an input terminal thereon and separating the RF signal into an I-channel and a Q-channel. The Q-channel signal separated by the quadrature hybrid coupler 11 is provided to a first biphase modulator 12, and the I-channel signal is provided to the second biphase modulator 13. The I and Q-channel signals have the same amplitudes, but with a 90° phase difference.
The first biphase modulator 12 modulates only a phase of a received Q channel signal in a range of 0° or 180°, and the second biphase modulator 13 modulates only a phase of a received I channel signal in a range of 0° or 130°. Therefore, most phase modulators 12 and 13 are biphase modulators which shift only the phases of the signals within a range of 0° or 180° and not the amplitudes of the signals. The first and second phase invariant attenuators 14 and 15 adjust only the amplitudes of the channel signals from the biphase modulators 12 and 13 according to their own ranges of attenuations, respectively. The combiner 16 combines signals from the first and second phase invariant attenuators 14 and 15 and forwards the combined signal through an output terminal. The attenuation ranges of the first and second phase invariant attenuators 14 and 15 are proportional to an entire range of variation of a vector modulator as shown in equation (1), below.R=10−(X/20)  (1) 
In equation 1, X is an attenuation value dB of each phase invariant attenuator and R is an output of the vector modulator. The attenuation ranges of respective phase invariant attenuators can be expressed as equations (2) and (3).I(dB)=20 log(R cos θ)  (2) Q(dB)=20 log(R sin θ)  (3) 
When the attenuation values dB of the phase invariant attenuators operative as equations (2) and (3) are varied up to 20 dB at 1 dB intervals, outputs of the vector modulator are as shown in FIG. 3. Therefore, as the attenuation ranges of the phase invariant attenuators 14 and 15 become greater, the ranges of variation of the vector modulator become greater in proportion to the attenuation ranges. Such a vector modulator is called as an I-Q vector modulator.
The I-Q vector modulator shown in FIG. 1 will be described. It is assumed that a RF signal with a phase angle of θ is provided to the I-Q vector modulator shown in FIG. 1. The quadrature hybrid coupler 11 provides an I channel signal with a phase angle θ′ through one path, and a Q channel signal with a phase angle of θ+90° through another path. The first biphase modulator 12 and the second biphase modulator 13 shift phases of the channel signals within ranges of 20° or 180°, respectively. Therefore, a range of phase of the Q channel signal which can be provided from the first biphase modulator 12 is θ′˜θ′+180°, and a range of phase of the I channel signal which can be provided from the second biphase modulator 13 is θ′+90°˜θ′+270°.
However, as shown in FIG. 3, the related art I-Q vector modulator controls the amplitude and the phase of a received RF signal within a limited range, which can be verified by determining whether the modulated signal, output of the I-Q vector modulator, covers all regions of a polar coordinate system. The related art I-Q vector modulator can not vary the amplitude and the phase of the received signal to cover all sections of the polar coordinate system. Further, in order to express a vector having a very small amplitude in the vicinities of the I and Q axes, the attenuation value should be very small requiring an attenuator with a great attenuation range or many steps of attenuators. Since an amplifier is required additionally to express a value outside of the existing I-Q vector modulator range, the related art vector modulator has a limited range of use.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.